Together the books, part of a 16-volume set on 18th century French literary criticism, would have racked up close to $100,000 in late fees. But staff decided to take pity on the secondhand book dealer who recently stumbled across the pair at a deceased estate auction in Queensland.

Buyer Yarran Jenkins discovered the ANU stamp inside the two books late last year and diligently sent them home to the university to reunite the set.

Head librarian Roxanne Missingham said it was still something of a mystery how the books found their way thousands of kilometres up north, after they were first checked out in March 1993 and marked as missing in 2007.

Late fees are usually capped at $220 after four weeks, but Ms Missingham said the library's $6 per day rate put the fine considerably higher for both books; close to $100,000.

"We've decided to waive the fees this time," she laughed. "We do that sometimes, we can be merciful."

Like many universities, students at ANU with outstanding library fines cannot graduate until they settle their accounts, but Ms Missingham said it did not appear that the borrower of the books ran into such a dilemma in the '90s.

The "donations" are among more than 1400 books that have been gifted to the library since a savage storm in February 2018 left its lower level - and much of the ANU's humanities collection - underwater.

An estimated 300,000 volumes costing millions of dollars were lost in the deluge when the nearby Sullivans Creek burst its banks, including parliamentary papers, manuscripts and microfilm.

"We've been dealing with the grief as well," Ms Missingham said.

"Some staff have spent years working with academics to select magnificent materials not easy to replace, it took years to get it all on the shelves, [the collection] had just turned 50."

For the next three months, fans and dehumidifiers ran around the clock to safely air out the bottom level, which will be refurbished and converted into study space in the coming months.

"We'll not have books [down there] ever again," Ms Missingham said.

Sodden books were loaded into minus 18-degree freezers to buy the team time to plan their preservation. More than a year on, Ms Missingham said the freezers were still full but the library was almost out of time to restore their contents.

Texts held by other Australian libraries would be the first pulled from the cold storage and "experimented on", she said, as librarians worked with expert restorers to dry out pages and remove mould.

The final book to come out of the frost will be a rare text on Aboriginal languages; one of about 35 damaged in the ANU's collection not held by any other library in the country.

"I'm hopeful we can save it," Ms Missingham said.

Replacements of other texts have since flooded in from as far afield as the UK and Canada. One ANU academic, Jane Simpson, donated more than 300 books from her late father's collection, spending hours poring through titles to replace what was lost.

Donald Simpson, a neurosurgeon and professor, had heard about the Chifley Library disaster shortly before he died in 2018, Professor Simpson said.

"All his life he was interested in history, and collected books," she said.